Base station and user equipment

ABSTRACT

Techniques for implementing MIMO communication in accordance with reciprocity based precoding are provided. The present disclosure relates to a base station for implementing multi-antenna transmission that includes a communication control unit configured to control multi-antenna transmission with user equipment and a channel state estimation unit configured to estimate a channel state with the user equipment based on an uplink reference signal from the user equipment. The communication control unit causes the user equipment to transmit the uplink reference signal in an uplink shared channel in an uplink subframe.

TECHNICAL FIELD

The present invention relates to a radio communication system.

BACKGROUND ART

In LTE (Long Term Evolution) standard, MIMO (Multiple-InputMultiple-Output) technique using multiple antenna ports issophisticated. In typical MIMO communication, user equipment estimates adownlink channel state based on downlink reference signals received froma base station and feeds the estimated downlink channel state as channelstate information (CSI) back to the base station. The base stationcontrols downlink beam forming based on feedback of the channel stateinformation. According to this approach, a feedback channel is necessaryfor feedback of the channel state information, and more resources forthe feedback channel must be reserved due to increase in the number ofusers and/or the number of antennas.

For the problem of resource reservation for the feedback channel,reciprocity based precoding is discussed. The reciprocity basedprecoding is an approach where abase station measures an uplink channelstate instead of measuring the downlink channel state to controldownlink beam forming based on the measured uplink channel state. Thisis based on assumption where the uplink channel state is almostidentical to the downlink channel state, and measurement results of theuplink channel state are used instead of the downlink channel state. Theuplink channel state is currently measured based on uplink referencesignals such as a sounding reference signal and a pilot signal from theuser equipment.

See 3GPP T536.213 V12.2.0 (2014-06) for further details, for example.

SUMMARY OF INVENTION Problem to be Solved by the Invention

In the case where the measurement results of the uplink channel stateare used instead of the downlink channel state, however, it is necessaryto transmit the sounding reference signals frequently. If each userequipment transmits the sounding reference signal frequently, asillustrated in FIG. 1, it is predicted that there is much influence ofinterference from user equipments visiting other cells. Particularly, anamount of transmitted sounding reference signals increases due toincrease in the number of users and the number of user antennas, thesophisticated MIMO techniques such as 3D MIMO using a large number ofantenna ports or the like, and it is considered that it may be difficultto reserve resources for the sounding reference signals. Also, it isconsidered that interference of uplink signals among user equipments maybe more significant.

Also, according to the reciprocity based precoding, a base station canselect a precoder or a precoding matrix indicator (PMI) based on theestimated uplink channel state. However, the base station cannotestimate channel quality or an interference state in the user equipment.

In light of the above-stated problem, an object of the present inventionis to provide techniques for implementing MIMO communication inaccordance with the reciprocity based precoding.

Means for Solving the Problem

In order to achieve the above object, one aspect of the presentinvention relates to a base station for implementing multi-antennatransmission, comprising: a communication control unit configured tocontrol multi-antenna transmission with user equipment; and a channelstate estimation unit configured to estimate a channel state with theuser equipment based on an uplink reference signal from the userequipment, wherein the communication control unit causes the userequipment to transmit the uplink reference signal in an uplink sharedchannel in an uplink subframe.

Another aspect of the present invention relates to user equipment,comprising: a communication control unit configured to control radiocommunication with a base station; and a channel state feedback unitconfigured to estimate a channel state with the base station based on areference signal received from the base station and feed the estimatedchannel state back to the base station, wherein the communicationcontrol unit transmits an uplink reference signal to the base station inan uplink shared channel in an uplink subframe.

Advantage of the Invention

According to the present invention, it is possible to implement the MIMOcommunication in accordance with the reciprocity based precoding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram for illustrating interference of pilotsignals;

FIG. 2 is a schematic diagram for illustrating a radio communicationsystem according to one embodiment of the present invention;

FIG. 3 is a block diagram for illustrating an arrangement of a basestation according to one embodiment of the present invention;

FIG. 4 is a diagram for illustrating exemplary multiplexing of referencesignals according to one embodiment of the present invention;

FIG. 5 is a diagram for illustrating exemplary multiplexing of referencesignals according to one embodiment of the present invention;

FIG. 6 is a diagram for illustrating exemplary multiplexing of referencesignals according to one embodiment of the present invention;

FIG. 7 is a diagram for illustrating exemplary multiplexing of referencesignals according to one embodiment of the present invention;

FIG. 8 is a diagram for illustrating exemplary multiplexing of referencesignals according to one embodiment of the present invention; and

FIG. 9 is a block diagram for illustrating an arrangement of userequipment according to one embodiment of the present invention.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are described below with referenceto the drawings.

In embodiments as stated below, abase station and user equipment aredisclosed for implementing MIMO communication by using reciprocity basedprecoding where downlink signals are precoded based on an uplink channelstate. Summarizing the embodiment below, the base station configuresuplink subframes for reference signals for uplink channel estimationbased on the reciprocity based precoding and causes the user equipmentto transmit uplink reference signals in an uplink shared channel(Physical Uplink Shared Channel: PUSCH) in an uplink subframe.Accordingly, the base station can use not only existing soundingreference signals but also the uplink reference signals transmitted inthe uplink shared channel to achieve accurate uplink channel estimationand determine a downlink precoder based on the estimation result.

Also, the base station sends the user equipment a downlink referencesignal (for example, a CSI-RS, a DM-RS and so on) to which the precoderdetermined in accordance with the reciprocity is applied, for example,and acquires channel quality (for example, an interference state) at theuser equipment as feedback information. Accordingly, the interferencestate at the user equipment, which cannot be estimated in accordancewith the reciprocity based precoding, can be obtained, and moreappropriate MIMO communication can be achieved.

At the outset, a radio communication system according to one embodimentof the present invention is described with reference to FIG. 2. FIG. 2is a schematic diagram for illustrating a radio communication systemaccording to one embodiment of the present invention.

As illustrated in FIG. 2, the radio communication system 10 has a basestation 100 and user equipment 200. The radio communication system 10 isan LTE system or an LTE-Advanced (LTE-A) system, but is not limited toit, and may be any radio communication system that support the MIMOcommunication.

The base station 100 implements the MIMO communication with the userequipment 200. The base station 100 particularly supports 3D MIMOcommunication and wirelessly connects for the user equipment 200 viamultiple antennas in incorporated multi-dimensional antennas such astwo-dimensional planar antennas and three-dimensional antennas.Specifically, the base station 100 transmits downlink (DL) packetsreceived from a network device such as an upper station and a servercommunicatively connected on a core network (not shown) to the userequipment 200 via multiple antenna ports and transmits uplink (UL)packets received from the user equipment 200 via the multiple antennaports to the network device.

The base station 100 is typically composed of hardware resources such asMIMO antennas for transmitting and receiving radio signals to/from theuser equipment 200, communication interfaces for communicating withadjacent base stations 100 (for example, an X2 interface), communicationinterfaces for communicating with the core network (for example, an S1interface) and a processor or circuit for processing transmission andreception signals with the user equipment 200. Functions and operationsof the base station 100 as stated below may be implemented by theprocessor running or processing data and/or programs stored in a memorydevice. However, the base station 100 is not limited to the above-statedhardware arrangement and may have any other appropriate hardwarearrangement. In general, a large number of base stations 100 aredisposed to cover a service area of the radio communication system 10.

The user equipment 200 implements the MIMO communication with the basestation 100 and transmits and receives radio signals such as variousdata signals and control signals to/from the base station 100 viamultiple antenna ports of the base station 100. In order to implementappropriate MIMO communication, the user equipment 200 estimates channelstates with respective antenna ports and feeds the estimated channelstates as channel state information (CSI) back to the base station 100.Upon receiving the channel state information, the base station 100controls the MIMO communication based on the received channel stateinformation.

The user equipment 200 may be typically any appropriate informationprocessing device having a radio communication function such as asmartphone, a cellular phone, a tablet, a mobile router and a wearableterminal. The user equipment 200 is composed of a CPU (CentralProcessing Unit) such as a processor, a memory device such as a RAM(Random Access Memory) and a flash memory, a radio communication devicefor transmitting and receiving radio signals to/from the base station100 and the like. For example, functions and operations of the userequipment 200 as stated below may be implemented by the CPU running orprocessing data and/or programs stored in the memory device. However,the user equipment 200 is not limited to the above-stated hardwarearrangement and may be composed of circuits for implementing one or moreof the operations as stated below.

Next, a base station according to one embodiment of the presentinvention is described with reference to FIGS. 3-8. FIG. 3 is a blockdiagram for illustrating an arrangement of the base station according toone embodiment of the present invention.

As illustrated in FIG. 3, the base station 100 has a communicationcontrol unit 110 and a channel state estimation unit 120. The basestation 100 implements MIMO communication and is particularly preferablefor 3D MIMO communication in accordance with the reciprocity basedprecoding.

The communication control unit 110 controls multi-antenna transmissionwith the user equipment 200. Specifically, the communication controlunit 110 assigns radio resources to the user equipment 200 and uses theassigned radio resources to transmit and receive radio signals to/fromthe user equipment 200 via multiple antenna ports in multi-antennatransmission. For example, in uplink transmission from the userequipment 200, the communication control unit 110 assigns radioresources (subcarriers, resource units or the like) in uplink subframesto the user equipment 200, and the user equipment 200 uses the assignedradio resources to transmit uplink signals. Also, in downlinktransmission to the user equipment 200, the communication control unit110 assigns radio resources (subcarriers, resource units or the like) indownlink subframes for transmission to the user equipment 200 andtransmits downlink signals to the user equipment 200 in the assignedradio resources.

The channel state estimation unit 120 estimates a channel state with theuser equipment 200 based on an uplink reference signal from the userequipment 200. Specifically, the channel state estimation unit 120 usesthe reciprocity based precoding to estimate an uplink channel statebased on the received uplink reference signal and controls downlink beamforming based on the estimated uplink channel state.

In order to implement appropriate MIMO communication in accordance withthe reciprocity based precoding, the channel state estimation unit 120has to receive uplink reference signals frequently and estimate anuplink channel state. In this embodiment, the communication control unit110 causes the user equipment 200 to transmit an uplink reference signalin an uplink shared channel in an uplink subframe. Specifically, inaddition to an existing sounding reference signal and/or uplinkdemodulation reference signal assigned to a predetermined domain in theuplink subframe, the communication control unit 110 causes the userequipment 200 to transmit the uplink reference signal in an uplinkshared channel domain in the uplink subframe.

In one embodiment, the communication control unit 110 may use timedivision multiplexing (TDM), frequency division multiplexing (FDM), codedivision multiplexing (CDM) or one or more combinations of the timedivision multiplexing, the frequency division multiplexing and the codedivision multiplexing to cause the user equipment 200 to transmit theuplink reference signal in the uplink shared channel in the uplinksubframe.

For example, as illustrated in FIG. 4, the communication control unit110 may assign the uplink shared channel in a predetermined uplinksubframe to the user equipments UE1 to UE 11 in according to the timedivision multiplexing and cause the uplink reference signal to betransmitted in the assigned uplink shared channel domain instead of anuplink data signal. As illustrated, a sounding reference signal (SRS)and an uplink demodulation reference signal (DM-RS) are assigned to apredetermined time domain in the uplink subframe, and the other domainsare assigned to the uplink shared channel to transmit the uplink datasignal. In this embodiment, however, a predetermined uplink subframe asa whole is assigned for transmission of the uplink reference signal asillustrated, and the uplink shared channel (domains other than the SRSand the DM-RS) in the uplink subframe is assigned to cause the userequipments UE1 to UE 11 to transmit the uplink reference signal.

Also, as illustrated in FIG. 5, the communication control unit 110 mayassign the uplink shared channel in a predetermined uplink subframe tothe user equipments UE1 to UE11 in accordance with a combination of thetime division multiplexing and the frequency division multiplexing andcause the uplink reference signal to be transmitted in the assigneduplink shared channel domain instead of the uplink data signal. Comparedto scheduling in FIG. 4, the respective user equipments UE1 to UE12transmit the uplink reference signals only in a part of a frequencydomain in each transmission time interval, and this subband transmissionmay be preferred from standpoint of limited power at the user equipments200.

Also, as illustrated in FIG. 6, the communication control unit 110 mayassign the uplink shared channel in a predetermined uplink subframe tothe user equipments UE1 to UE12 in accordance with a combination of thetime division multiplexing and the frequency division multiplexing andcause the uplink reference signal to be transmitted in the assigneduplink shared channel domain instead of the uplink data signal. Comparedto scheduling in FIG. 5, the respective user equipments UE1 to UE12transmit the uplink reference signals only in a part of a frequencydomain in each transmission time interval and transmit the uplinkreference signals in the whole frequency domain in the whole timedomain. In general, there are cases where the transmission power maybecome insufficient in uplink, but in this example, the whole systemband is covered while reducing influence of limited power at the userequipment 100. Accordingly, the subband transmission may be morepreferable.

Also, as illustrated in FIG. 7, the communication control unit 110 mayassign the uplink shared channel in a predetermined uplink subframe tothe user equipments UE1 to UE12 in accordance with a combination of thetime division multiplexing, the frequency division multiplexing and thecode division multiplexing and cause the uplink reference signal to betransmitted in the assigned uplink shared channel domain instead of theuplink data signal. Compared to scheduling in FIG. 6, the respectiveuser equipments UE1 to UE12 transmit the uplink reference signals onlyin a part of a frequency domain in each transmission time interval andtransmit the uplink reference signals in the whole frequency domain inthe whole time domain. Furthermore, the respective user equipments UE1to UE12 transmit the uplink reference signals in multiple transmissiontime periods in accordance with the code division multiplexing withoutreducing the number of assigned user equipments. In general, an RF errorsuch as a time error and a frequency error arises in a radio device, andit may be preferable for estimation and reduction in influence of the RFerror to transmit the uplink reference signal in multiple transmissiontime periods. Similarly, the respective user equipments UE1 to UE12transmit the uplink reference signals in multiple frequency positions,but it may be preferable for estimation and reduction in the influenceof the RF error to transmit the uplink reference signals in the multiplefrequency positions.

In one embodiment, the communication control unit 110 may cause the userequipment 200 to transmit an uplink reference signal in the whole or apart of an uplink shared channel in an uplink subframe. In the examplesas illustrated in FIGS. 4-7, the communication control unit 110 causesthe user equipment 200 to transmit the uplink reference signal in thewhole uplink shared channel domain in a predetermined uplink subframe.On the other hand, as illustrated in FIG. 8, the communication controlunit 110 may cause the user equipment 200 to transmit an uplink datasignal in a partial domain of the uplink shared channel other thandomains for the sounding reference signal (SRS) and the uplinkdemodulation reference signal (DM-RS). In other words, the userequipment 200 may transmit the uplink data signal in the other domainsof the uplink shared channel.

Also, the communication control unit 110 may cause the user equipment200 to transmit the uplink reference signal in not only the uplinkshared channel but also domains for the sounding reference signal and/orthe uplink demodulation reference signal. Particularly, if the uplinkdata signal is not transmitted in the uplink shared channel, it is notso necessary to transmit the uplink demodulation reference signal, andthe uplink demodulation reference signal domain may be assigned fortransmission of the uplink reference signal.

Also, the communication control unit 110 may indicate radio resourcesfor the uplink reference signal in RRC (Radio Resource Control) and/or(e)PDCCH (Physical Downlink Control Channel). Specifically, thecommunication control unit 110 may specify the radio resources for theuplink reference signal with a position in a time domain (a transmissiontime interval or a symbol), a position in a frequency domain (a resourceblock or a subcarrier), a bandwidth, a cyclic shift (CS) index, a timingoffset, a cycle for cyclic transmission or the like.

In one embodiment, the communication unit 110 may periodically assign anuplink subframe to cause the user equipment 200 to transmit an uplinkreference signal in an uplink shared channel. Predetermined uplinksubframes for transmission of the uplink reference signals as statedabove with reference to FIGS. 4-8 may be periodically configured. Forexample, the communication control unit 110 may assign uplink subframesfor the reference signal for each 20 milliseconds.

On the other hand, the communication control unit 110 may aperiodicallyassign uplink subframes to cause the user equipment 200 to transmit theuplink reference signal in the uplink shared channel. Predetermineduplink subframes for transmission of the uplink reference signals asstated above with reference to FIGS. 4-8 may be aperiodicallyconfigured. For example, the communication control unit 110 may triggertransmission of the reference signal in the (e)PDCCH.

In the above-stated embodiments, the communication control unit 110increases an amount of transmitted uplink reference signals by causingthe user equipment 200 to transmit the uplink reference signal in theuplink shared channel. In other embodiments, the communication controlunit 110 may adjust the number of user equipments 200 to transmit thesounding reference signal by adjusting a sampling factor of the soundingreference signal. Specifically, the existing sounding reference signalis inserted in a cycle of every two subcarriers, and the insertion cyclemay be set to a greater value. Accordingly, the channel state estimationunit 120 can receive the sounding reference signal from a larger numberof user equipments 200 and estimate the uplink channel states with theuser equipments 200 based on the sounding reference signals receivedfrom the larger number of user equipments 200.

Furthermore, the sounding reference signal may be aperiodic or periodic.Also, in order to cause the sounding reference signal to be transmittedin one precoded stream rather than multiple streams, the communicationcontrol unit 110 may indicate a PMI for the sounding reference signal tothe user equipment 200. On the other hand, the user equipment 200 mayautonomously select the PMI for the sounding reference signal andindicate it to the base station 100. Also, the PMI for the soundingreference signal may be the same as one applied to the uplink sharedchannel.

Also, in order to improve accuracy of the sounding reference signal, thecommunication control unit 110 may indicate transmission power of thesounding reference signal to the user equipment 200. In the current LTEstandard, the transmission power for the sounding reference signal isset to an offset value (P_(SRS) _(_) _(OFFSET, c)(m)) from thetransmission power applied to the uplink data channel. In order toimprove channel estimation accuracy with the reciprocity based precoding, it is desired that the sounding reference signal is transmittedat higher power. To this end, the communication control unit 110 may setthe transmission power of the sounding reference signal individually.Furthermore, the offset value is fixed for a PUSCH for link adaptationcontrol of uplink but does not have to be defined as the offset fromstandpoint of the reciprocity. Furthermore, since it is undesirable thatthe power changes without expectation by the base station, it is desiredthat the transmission power is determined independently of transmissionpower control of the PUSCH, for example. The discussion of thetransmission power can be applied to not only the sounding referencesignal but also the other uplink reference signals.

Also, the communication control unit 110 may pre code and transmit adownlink reference signal (CSI-RS) for measuring downlink channelstates. Also, the communication control unit 110 may indicate a rank forthe precoded downlink reference signal. The communication control unit110 may spatially multiplexes the downlink reference signal to estimateinter-user interference. Also, the communication control unit 110 maytransmit the downlink reference signal in an existing reference signaldomain or a data domain. Also, the sounding reference signal and thedownlink reference signal may be precoded with the same codebook.

Next, the user equipment according to one embodiment of the presentinvention is described with reference to FIG. 9. FIG. 9 is a blockdiagram for illustrating an arrangement of the user equipment accordingto one embodiment of the present invention.

As illustrated in FIG. 9, the user equipment 200 has a communicationcontrol unit 210 and a channel state feedback unit 220. The userequipment 200 implements MIMO communication and is particularlypreferable for 3D MIMO communication in accordance with the reciprocitybased precoding.

The communication control 210 controls radio communication with the basestation 100. Specifically, in downlink communication, the communicationcontrol unit 210 receives downlink radio signals transmitted frommultiple antenna ports in the base station 100 with beams precoded inthe horizontal direction, the vertical direction or the like and uses acodebook for use in the beam control to demodulate the received radiosignals. Also, in uplink communication, the communication control unit110 uses the codebook to transmit uplink radio signals to the basestation 100.

The channel state feedback unit 220 estimates a channel state with thebase station 100 based on the reference signal received from the basestation 100 and feeds the estimated channel state back to the basestation 100. As stated above, according to the reciprocity basedprecoding, the base station 100 can select a codebook or a precodingmatrix indicator (PMI) based on the estimated uplink channel state butcannot estimate an interference state, that is, a CQI at the userequipment 200. Accordingly, in order to achieve appropriate MIMOcommunication, the channel state feedback unit 220 may estimate achannel quality indicator (CQI) based on the reference signaltransmitted from the base station and feed the estimated CQI back to thebase station 100. In other words, the base station 100 can receive theCQI and/or the rank indicator (RI) as feedback information from the userequipment 200 and know the interference state at the user equipment 200.

In this embodiment, the communication control unit 210 transmits anuplink reference signal to the base station 100 in an uplink sharedchannel in an uplink subframe. In order to achieve the appropriate MIMOcommunication in accordance with the reciprocity based precoding, thebase station 100 has to receive the uplink reference signal frequentlyand estimate the uplink channel state. Accordingly, in this embodiment,the communication control unit 210 transmits the uplink reference signalto the base station 100 in an uplink shared channel domain in the uplinksubframe in addition to an existing sounding reference signal and/oruplink demodulation reference signal assigned to a predetermined domainin the uplink subframe. For example, the communication control unit 210may transmit the uplink reference signal in the uplink shared channel ina predetermined uplink subframe for reference signal periodicallyassigned by the base station 100.

In one embodiment, the channel state feedback unit 220 may estimate achannel state with the base station 100 based on a precoded CSI-RS or aprecoded downlink DM-RS. According to the conventional MIMOcommunication, the user equipment 200 receives a non-precoded CSI-RS,selects an appropriate codebook based on the received CSI-RS and feeds aPMI for the selected codebook as well as a CQI and/or a RI as channelstate information (CSI) back to the base station 100. Meanwhile,according to the reciprocity based precoding, the base station 100selects a codebook or a PMI based on the uplink channel state andtransmits radio signals, which are beam controlled under the selectedcodebook, to the user equipment 200. In other words, the user equipment200 receives the downlink reference signal precoded with the codebookselected by the base station 100 from the base station 100.

Note that according to the reciprocity based precoding, the base station100 selects the codebook or the PMI based on the estimated uplinkchannel state. The base station 100 would transmit the radio signalsbeam controlled under the selected codebook to the user equipment 200without the user equipment 200 knowing which of codebooks has beenselected. In general, the CQI measured at the user equipment 200 changesdepending on a beam control state of the downlink reference signal.Accordingly, it is preferred that the user equipment 200 knows which ofcodebooks is applied to the received downlink reference signal. To thisend, the channel state feedback unit 220 may acquire the PMI for thecodebook applied to the received precoded downlink reference signal fromthe base station 100. Alternatively, the codebook applied to thedownlink reference signal may be autonomously selected by the userequipment 200.

Also, if the channel state information estimated based on the downlinkDM-RS is fed back to the base station 100, the channel state feedbackunit 220 may transmits the channel state information to the base station100 together with ACK/NACK indicative of whether the downlink datasignal has been successfully received.

In one embodiment, the communication control unit 210 uses a part ofantennas in the user equipment 200 to transmit the sounding referencesignal to the base station 100. Here, the reference signal received fromthe base station 100 may be precoded based on the sounding referencesignal received at the base station 100. The sounding reference signaldoes not have to be transmitted with all the antennas in the userequipment 200 and may be transmitted from only a part of the antennassuch as one antenna for reduction in overhead. It is often sufficient touse only the single antenna to estimate channel characteristics such asan AoD (Angular of Departure), an AoA (Angular of Arrival), a ZoD(Zenith angle of Departure) and a ZoA (Zenith angle of Arrival). Thenumber of antennas to transmit the sounding reference signal may beindicated by the base station 100. Also, if the sounding referencesignal is transmitted from multiple antennas, a gain difference betweenthe respective antennas may be compensated for. For example, thetransmission power of an antenna having a relatively low gain may beincreased.

Also, the communication control unit 210 may transmit the precodedsounding reference signal to the base station 100. It is considered thatthe precoded sounding reference signal may be received at the basestation 100 at higher reception power than the not-precoded soundingreference signal, and it is also considered that the precoded soundingreference signal may reduce influence of interference from other userequipments 200.

According to the base station 100 and the user equipment 200 as statedabove, the user equipment 200 first transmits the sounding referencesignal in radio resources assigned by the base station 100. The basestation 100 selects an optimal downlink precoder or codebook based onthe received sounding reference signal and transmits a downlinkreference signal precoded with the selects downlink precoder to the userequipment 200. The user equipment 200 feeds a CQI based on the receivedprecoded downlink reference signal back. In this manner, the basestation 100 can know interference (inter-cell interference, inter-userinterference or the like) at the user equipment 200, which cannot beestimated in accordance with the reciprocity based precoding.

Although the embodiments of the present invention have been described indetail, the present invention is not limited to the above-statedspecific embodiments, and various modifications and variations can bemade within the spirit of the present invention as recited in claims.

This international patent application claims benefit of priority basedon Japanese Priority Application No. 2014-195885 filed on Sep. 25, 2014,the entire contents of which are hereby incorporated by reference.

LIST OF REFERENCE SYMBOLS

-   -   10: radio communication system    -   100: base station    -   200: user equipment

1. A base station for implementing multi-antenna transmission, comprising: a communication control unit configured to control multi-antenna transmission with user equipment; and a channel state estimation unit configured to estimate a channel state with the user equipment based on an uplink reference signal from the user equipment, wherein the communication control unit causes the user equipment to transmit the uplink reference signal in an uplink shared channel in an uplink subframe.
 2. The base station as claimed in claim 1, wherein the communication control unit uses time division multiplexing, frequency division multiplexing, code division multiplexing or one or more combinations of the time division multiplexing, the frequency division multiplexing and the code division multiplexing to cause the user equipment to transmit the uplink reference signal in the uplink shared channel in the uplink subframe.
 3. The base station as claimed in claim 1, wherein the communication control unit causes the user equipment to transmit the uplink reference signal in a whole or part of the uplink shared channel in the uplink subframe.
 4. The base station as claimed in claim 1, wherein the communication control unit assigns the uplink subframe periodically to cause the user equipment to transmit the uplink reference signal in the uplink shared channel.
 5. The base station as claimed in claim 1, wherein the communication control unit adjusts a number of user equipments to transmit a sounding reference signal by adjusting a sampling factor of the sounding reference signal.
 6. The base station as claimed in claim 5, wherein the communication control unit indicates transmission power for the sounding reference signal to the user equipment.
 7. User equipment, comprising: a communication control unit configured to control radio communication with a base station; and a channel state feedback unit configured to estimate a channel state with the base station based on a reference signal received from the base station and feed the estimated channel state back to the base station, wherein the communication control unit transmits an uplink reference signal to the base station in an uplink shared channel in an uplink subframe.
 8. The user equipment as claimed in claim 7, wherein the channel state feedback unit estimates the channel state with the base station based on a precoded CSI-RS (Channel State Information-Reference Signal) or a precoded downlink DM-RS (Demodulation-Reference Signal).
 9. The user equipment as claimed in claim 7, wherein the channel state feedback unit estimates a CQI (Channel Quality Indicator) based on the received reference signal and feeds the estimated CQI back to the base station.
 10. The user equipment as claimed in claim 7, wherein the communication control unit uses a part of antennas of the user equipment to transmit a sounding reference signal to the base station.
 11. The base station as claimed in claim 2, wherein the communication control unit causes the user equipment to transmit the uplink reference signal in a whole or part of the uplink shared channel in the uplink subframe.
 12. The base station as claimed in claim 2, wherein the communication control unit assigns the uplink subframe periodically to cause the user equipment to transmit the uplink reference signal in the uplink shared channel.
 13. The base station as claimed in claim 3, wherein the communication control unit assigns the uplink subframe periodically to cause the user equipment to transmit the uplink reference signal in the uplink shared channel.
 14. The base station as claimed in claim 2, wherein the communication control unit adjusts a number of user equipments to transmit a sounding reference signal by adjusting a sampling factor of the sounding reference signal.
 15. The base station as claimed in claim 3, wherein the communication control unit adjusts a number of user equipments to transmit a sounding reference signal by adjusting a sampling factor of the sounding reference signal.
 16. The base station as claimed in claim 4, wherein the communication control unit adjusts a number of user equipments to transmit a sounding reference signal by adjusting a sampling factor of the sounding reference signal.
 17. The user equipment as claimed in claim 8, wherein the channel state feedback unit estimates a CQI (Channel Quality Indicator) based on the received reference signal and feeds the estimated CQI back to the base station.
 18. The user equipment as claimed in claim 8, wherein the communication control unit uses a part of antennas of the user equipment to transmit a sounding reference signal to the base station.
 19. The user equipment as claimed in claim 9, wherein the communication control unit uses a part of antennas of the user equipment to transmit a sounding reference signal to the base station. 